如何在C＃中验证数据读取器 [英] How to validate data reader in C#

问题描述

 从学生中选择 *  

 Rollno Studname年龄DOB地址有效状态
 1 Ajay 6 12.10.2010 Chennai A Male 
 2 Suresh 5 5.7.2011 Chennai A Male 
 3 Priya 6 10.6.2010 Chennai A FeMale 
 4 Revathy 5 8.9.2011 Chennai A FeMale 
 

来自上表我的验证如下

 使用（SqlConnection newCon =  new  SqlConnection（db.GetConnectionString））
 {
 SqlCommand newCmd =  new  SqlCommand（从学生中选择* ） ，newCon）; 
 newCmd.Parameters.Add（）
 newCon.Open（）; 
 SqlDataReader rdr = newCmd.ExecuteReader（）; 
 rdr.Read（）; 
 
 if（reader [6] .ToString（）== Male  ）; 
 {
 //我正在显示男性形象
} 
 if（reader [6] .Tostring（）== FeMale  ）; 
 
 {
 //我正在显示FeMale图片
} 
 
}  

当我执行上述代码时显示错误如下



无效尝试没有数据时读取。



以上错误显示如下以下

  if （reader [ 6 ]。ToString（）==  男性）

我上面代码中的错误是什么。请帮帮我



i试用很多但显示错误。

解决方案

你从DataReader读取，但是你不要检查是否返回任何行：所以当它们不是时，你会得到一个明确告诉你的错误！

首先检查返回值：

  if （rdr.Read（）; 
 {
  if （读者[ 6 ]。ToString（）==  男性）; 
 {
 ... 

并且即时错误将消失。



但是...你显示的代码不是你正在运行的代码，因为这不会编译：

 SqlCommand newCmd =  new  SqlCommand（从学生中选择* ） < span class =code-string>，newCon）;  

我怀疑你的真实代码正在使用WHERE子句来选择返回信息的学生。并且WHERE子句不起作用...所以没有返回任何行。

我建​​议创建 Dictionary [ ^ ]对象具有以下功能：

字典<字符串，字符串> images =  new 字典< string，string>（）; 
 images.Add（ 男性，   C：\ Pictures\Male.jpg）; 
 images.Add（  FeMale，   C：\ Pictures\FeMale.jpg）; 

然后，您将能够根据学生的性别简单地指定图像路径。

 PictureBox1.Image = Image.FromFile（images [reader [  6 ]]）; 

详情请见：

在运行时设置图片（Windows窗体） [ ^ ]

Image.FromFile Method（字符串）（System.Drawing） [ ^ ]

Select * from student

Rollno      Studname   Age    DOB            Address   Active    Status
   1          Ajay      6     12.10.2010      Chennai   A          Male
   2          Suresh    5     5.7.2011        Chennai   A          Male
   3          Priya     6     10.6.2010       Chennai   A         FeMale
   4          Revathy   5     8.9.2011        Chennai   A         FeMale

from the above table i am validating as follows

using (SqlConnection newCon = new SqlConnection(db.GetConnectionString))
            {
                SqlCommand newCmd = new SqlCommand(Select * from student)", newCon);
                newCmd.Parameters.Add()
                newCon.Open();
                SqlDataReader rdr = newCmd.ExecuteReader();
                rdr.Read();

                if(reader[6].ToString() == "Male");
                    {
                          //In that i am displaying Male Image
                    }
                 if(reader[6].Tostring() == "FeMale");
                    
                     {
                          //In that i am displaying FeMale Image
                     }
                   
           }

When i exeute the above code shows error as follows

Invalid attempt to read when no data is present.

The above error shows in below line as follows

if(reader[6].ToString() == "Male")

What is the mistake in my above code. please help me

i tried lot but shows error.

解决方案

You read from the DataReader, but you don't check that any rows are returned: so when they aren't you get an error which explicitly tells you just that!
Start by checking the return value:

if (rdr.Read();
   {
   if(reader[6].ToString() == "Male");
      {
   ...

And the immediate error will go away.

But...the code you show isn't the code you are running, because this won't compile:

SqlCommand newCmd = new SqlCommand(Select * from student)", newCon);

And I suspect that your "real" code is using a WHERE clause to select which student to return info for. And that WHERE clause doesn't work...so no rows are returned.

I'd suggest to create Dictionary[^] object for such of functionality:

Dictionary <string, string> images = new Dictionary<string, string>();
images.Add("Male", "C:\Pictures\Male.jpg");
images.Add("FeMale", "C:\Pictures\FeMale.jpg");

Then you'll be able to simply assign path to image depending on student's gender.

PictureBox1.Image = Image.FromFile(images[reader[6]]);

For further details, please see:
Setting Pictures at Run Time (Windows Forms)[^]
Image.FromFile Method (String) (System.Drawing)[^]

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